A radiometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Because electromagnetic radiation can be correlated to temperature, the radiometer is generally a popular choice in sensing thermal radiation in many different applications (e.g., terrestrial and extra-terrestrial sensing, medical diagnostics, defense applications, weather, and so on). Even though the term radiometer is often used to denote an infrared radiation detector, the designation radiometer also includes detectors operating on any electromagnetic wavelength.
While several types of radiometric devices are available, the comparison type of radiometer is generally the most widely used. One particular type of comparison radiometer, often referred to as a Dicke radiometer, uses the microware/millimeter wave frequency bands (i.e., the MMW radiometer) and compares an incoming signal to a standard or calibrated reference signal, which is based on a known temperature. By comparing the incoming signal to the reference signal, the temperature of the scene that was sensed can be determined.
Depending on the particular application for which it will be used, the comparison type of radiometer may be implemented in a microwave monolithic integrated circuit (MMIC) or through discrete implementation using printed hybrids and multiple low noise amplifiers (LNAs).
Unfortunately, the comparison type of radiometer employs a radio frequency (RF) switch to toggle between the two available inputs. The RF switch may be used to switch between one input corresponding to the sensed scene and another input corresponding to the reference load. However, since RE switches are lossy devices, they may significantly and undesirably diminish the low noise performance of the radiometer. Eliminating the switch loss, while preserving input selectivity, will improve radiometric performance.